(1) Field of the Invention
This invention relates to a phase shifter device for use at microwave or radio frequencies, and more particularly to phased array antenna apparatus incorporating a plurality of such phase shifter devices.
(2) Description of the Art
Phased array antennas are well known. In such devices the resultant radiation pattern transmitted (the transmit beam), or the radiation pattern received (the receive beam), is controlled by variation of the relative phase of the signal transmitted, or received, by each antenna element that forms the phased array. In this manner, it is possible to steer a transmit or receive beam electronically without any mechanical movement of the antenna.
The phase differences necessary for phased array operation can be introduced in a variety of ways. One of the simplest techniques is to link each antenna element to a common feed point using co-axial cables of different length. However, this fixes the relative phase shifts between the antenna elements which may be undesirable in certain applications where phase adjustment is required. It is also possible to obtain accurate phase control at very high speeds over multiple element phased array antenna using digital electronic phase shifting devices of the type used in phased array radar systems. Although such an approach is highly suited to radar applications, the complexity and cost of the electronic circuitry is prohibitively expensive for low cost, mass market, applications.
In cases where rapid phase control is not required, but providing a fixed relative phase difference is insufficient, low complexity adjustable phase shifters have been developed. Such phase shifters enable control, for example by an engineer setting up a system, over the directionality of phased array antennas having a relatively low number of elements. US2002/0003458 and JP6326501 describe such phase shifters.
In US2002/0003458 a phase shifter is described in which a dielectric element having a number of teeth is moveably mounted over a pattern of conductive tracks formed on a planar dielectric circuit board. Movement of the dielectric element alters the propagation velocity through the conductive track, thereby imparting the required phase shift. This enables a phase shift to be introduced between sets of transmitter elements (typically two or three) to enable a downward tilt of the transmit and receive beams of a cellular telephone base station transceiver. A drawback of the device of US2002/0003458 is that the linear movement of the moveable dielectric element does not provide a correspondingly linear change in the phase shift imparted to the signal as the effect of the dielectric element is difficult to predict. This can make it difficult to control the amount of phase shift imparted to a signal.
JP6326501 describes a variable phase shifter having a substrate that is provided with a pair of arc shaped slot lines of different radius. An output terminal is connected to each end of each slot line. A rotatable arm distributes input radiation to each of the arc shaped slot lines, and this radiation is further distributed to each of the four slot line output terminals. Rotation of the arm alters the relative path length between the input radiation and each of the four output terminals, thereby altering the phase of the radiation at each of the output terminal. A disadvantage of the device of JP6326501 is that it is only possible to operate the device as a combined signal splitter and phase shifter. Furthermore, independent control of the phase shift imparted to each of the four output signals is not possible; the geometry of the device dictates the phase shifts imparted to each of the four output signals for a given orientation of the rotatable arm.